Xylem and Phloem Cells in Plants

Introduction 

Food and water are the most basic requirements of life in any organism. Plants also these nutrients for survival and growth. In animals, the circulatory system performs the function of the transportation of nutrients. In plants, for the transportation of food and water vascular tissues are present known as Xylem and Phloem. These tissues are present together in the form of vascular bundles. These bundles establish a connection between its different parts i.e. leaves, stems, and roots. These tissues transport water, minerals, and nutrients in the plant that are essential to maintain life. On the basis of the presence and absence of vascular tissues plants are divided into two types: 

  1. Vascular plants 
  2. Non-vascular plants 

Vascular plants 

Those plants that contain vascular tissues (xylem and phloem). Most of the plants are vascular plants. 

Non-vascular plants 

Plants that do not have xylem and phloem are known as non-vascular plants. Some simple plants such as mosses do not have vascular tissues and are known as non-vascular plants. 

Xylem 

Xylem is a type of vascular tissue that is dead and permanent. It is involved in the transportation of water and minerals from roots to all other parts of plants. The term ‘xylem’ comes from the Greek word ‘xylon’ which means wood. These cells are elongated and hollow. 

In these cells, both the nucleus and cytoplasm are not present. The end walls of cells are broken to form a long pathway for the transport of water and mineral salts. The mixture of the materials that flow through the xylem is known as sap. 

In mature xylem vessels: 

  • No cytoplasm is present. 
  • Become impermeable to water. 
  • The walls become thick due to the presence of woody material lignin. 

Types of xylem 

On the basis of growth, there are two types of xylem:

  • Primary xylem 
  • Secondary xylem 

Primary xylem 

Primary xylem formed as a result of the primary growth of the plant. The primary growth of the plant takes place at the tips of stems, roots, and flower buds. This type of growth facilitates the plant to grow taller and the roots to grow longer. This growth is known as primary growth of a plant because it occurs first when the plant grows in the growing season before secondary growth. 

Secondary xylem

When the plant shows secondary growth after primary growth, then secondary xylem is formed. Secondary growth allows the plant to become wider over time. For example, older trees have wide tree trunks due to a lot of secondary growth. Due to secondary xylem, dark rings are formed inside the tree trunks. The rings are useful to determine the age of a tree. It contains conducting cells. 

Both primary and secondary xylem perform the same function. 

Cells of xylem

Two types of cells are responsible for the formation of xylem in different plants. These cells include: 

  • Tracheids
  • Vessel elements 

Tracheids

These cells are connected together by tarped ends and have long and thin structure. The tarped ends are present parallel to each other and contain pits (areas in xylem where the secondary wall is not present) for the transportation of water from cell to cell. 

The secondary wall of tracheids contains lignin- the compound responsible for the formation of wood. This lignin in tracheids is responsible for structural support to xylem and complete plant. 

Gymnosperms, ferns, and lycophytes mostly contain tracheids. 

Vessel elements

Vessel elements are connected end to end and are structurally smaller and wider than tracheids. The end of vessel element cells contains “perforation plates”. There are many holes in the cell walls of perforation plates that help the water to move freely between the cells. 

Vessel elements form the xylem of most of the angiosperms. 

Phloem 

Phloem is also a type of vascular tissue that is living and permanent in nature. It is involved in the transportation of organic nutrients such as sugars from leaves (source-point of synthesis) to all other parts of plants (sink). The word phloem comes from the Greek word ‘phloios’ which means ‘bark’. 

The major function of phloem is to transport the products of photosynthesis (soluble organic compounds) to different parts of plants where they are required. This process is known as translocation. In addition to the transportation of sugars and amino acids, the process of translocation also allows the movement of pesticides in plants. 

When phloem cells mature, they do not contain a nucleus. Few organelles are present, and they depend on companion cells for fulfilling their metabolic requirements. 

Types of phloem

Phloem grows completely in two phases like xylem. Thus, on the basis of growth, phloem is divided into two types: 

  • Primary phloem 
  • Secondary phloem

Primary phloem 

It is formed during the early developmental stages of a plant known as primary growth. Primary phloem is formed from the apical meristems and develops from procambium. 

Secondary phloem

Secondary is formed from the vascular cambium on the inside of the layer of phloem. Like secondary xylem, it contains conducting cells for the transport of materials. 

Cells of Phloem 

Phloem is made from the following cells: 

  • Sieve tube elements 
  • Companion cells
  • Parenchyma cells 
  • Sclerenchyma cells 
  • Sieve plates 

Sieve tube elements 

Sieve tube elements are the living cells of plants that create long chains of cells present throughout the plant. In angiosperms, the sieve-tube elements have porous ends that are known are ‘sieve plates’. Due to these sieve plates, the diffusion of sap from cell to cell becomes easy. 

In sieve-tube cells, some important structures are not present. These structures include a nucleus, ribosomes, and a vacuole. Companion cells contain these structures. 

Companion cells 

Companion cells are located parallel to the sieve-tube members and are connected with each other by a number of connecting channels known as ‘plasmodesmata’. They contain all the essential organelles, and their nucleus and ribosomes are used by both the sieve-tube members and itself. Sometimes, companion cells are also responsible for transporting sugars and other substances from the surrounding cells to the sieve-tube members. 

Parenchyma cells 

Parenchyma cells are made up of thin and flexible walls of cellulose. Parenchyma provides support to the soft tissues of plants. In phloem, they are basically involved in the storage of starch, fats, and proteins. In some plants, they store tannins and resins as well. 

Sclerenchyma cells 

Sclerenchyma is the major support system of the phloem and gives it stiffness and strength. Sclerenchyma is of two types: 

  • Fibers 
  • Sclereids 

Both the fibers and sclereids have a thick secondary cell wall and become dead when they become mature. 

Sieve plates 

Sieve plates are the modified plasmodesmata that serve as connections between sieve tube elements. Sieve plates are large and thin and help in the transport of materials between the sieve tube elements. 

Sieve plates also serve as a barrier to avoid the loss of sap if the phloem gets damaged or cut. 

Major characteristic functions of Xylem and Phloem 

Xylem 

The basic function of xylem is the transport of water and salts from the roots to the other parts of plants. The transport of sap through xylem occurs by passive transport, so the process can occur in the absence of energy. The phenomenon through which the sap moves in the upward direction against gravity is known as capillary action. 

The water transported by xylem from roots to other parts of plants helps the plant to overcome the water loss due to transpiration and photosynthesis. 

Phloem

Phloem is involved in the transport of organic compounds such as sugars produced during photosynthesis from source to sink. This process is known as translocation. The movement of materials in phloem is by active transport. 

Movement of fluid 

Xylem 

In xylem, the driving force for the movement of water and salts is the rate of transpiration from leaves along with the cohesion and tension of water in the non-living cells i.e. vessel elements and tracheids. Negative pressure potential is exerted from the top. 

Phloem 

The transport of fluid in phloem occurs by active transport which requires energy. The fluid passes through the living phloem tube elements with the expenditure of energy in form of ATP. The pressure potential in the phloem is positive due to the push from the source. 

Structure 

Xylem 

Xylem is made up of different types of cells. The most important cells are tracheids that are responsible for the transport of xylem sap and also provide mechanical support to the plant. In addition, vessel elements are present that are also involved in the transport of water. Vessel elements in xylem are connected with each other to form a continuous plate by perforation plates. Parenchyma tissue is also present in xylem. This tissue forms mostly the soft structures of plants, long fibers, and provide support to the plant. When seen under an electron microscope, the structure of xylem appears to be star shaped. 

Phloem 

Phloem structure is made up of several different components. All these components work together to facilitate the transportation of sugars and amino acids from the site of synthesis (source) to the sites of storage and consumption (sink). These components include companion cells, sieve tube elements, and parenchyma cells. 

Differences between xylem and phloem 

The following are some of the major differences in xylem and phloem of the plants. 

Xylem

  1. Xylem is a complex vascular tissue that is involved in the transportation of water from roots to stems and leaves. It is also involved in the transport of nutrients. 
  2. Xylem tissue is present in the middle of the vascular bundle. 
  3. The movement of water and nutrients in xylem is unidirectional. 
  4. It is mainly composed of dead cells. Parenchyma cells are the only living cells that are present in xylem. 
  5. The xylem cells are responsible for providing mechanical support to the plants. 
  6. When xylem becomes mature, it loses all the cell content and becomes dead tissue. 
  7. Xylem cells consist of xylem fibers, vessel elements, and tracheids. 
  8. In xylem, due to the thickening of the wall, the conducting or tracheary cells become dead. 
  9. Xylem is often a major portion of the plant body. 
  10. In xylem, the conducting channels (vessels) lack septa.
  11. The thickenings of tracheary elements of xylem are different. 
  12. In mature plants, the xylem is known as heart wood and sap wood after differentiation. 
  13. Xylem favors the formation of tyloses. 
  14. Xylem fibers are mostly smaller in size. 

Phloem  

  1. Phloem is a complex vascular tissue in plants that is living. It is involved in the transport of organic compounds such as sugars and amino acids that are produced during photosynthesis in the leaves. 
  2. Phloem is located on the outer side of the vascular bundle. 
  3. The movement of substances in the phloem is bidirectional. 
  4. Phloem is mainly composed of living cells and the only dead cells in phloem are fibers. 
  5. They do not provide mechanical support to the plants. 
  6. When phloem cells mature, they are living tissues but do not contain a nucleus. 
  7. The basic elements of phloem are fibers, sieve tubes, sieve cells, parenchyma, and companion cells. 
  8. Lignin is not present in the conducting cells or sieve tubes because they are living structures. 
  9. Phloem constitutes a smaller portion of plant. 
  10. The septa of the sieve tubes are usually bulging and porous. 
  11. In conducting channels, wall thickenings are not present. 
  12. Phloem does not undergo much differentiation. 
  13. In phloem, the formation of tyloses does not take place. 
  14. The fibers of phloem are usually known as blast fibers because they are larger in size. 

Similarities of xylem and phloem

  1. Both are vascular tissues and involved in the transportation of substances. 
  2. Both are complex made up of more than one type of cell. 
  3. Living parenchymatous cells are components of both xylem and phloem. 
  4. Blast fibers, dead cells, surround xylem as well as phloem. 
  5. Both primary and secondary growth is found in xylem and phloem. 
  6. The precursor of both tissues is procambium that remains undifferentiated during primary xylem and phloem formation. 

Xylem and phloem are thus important structures that help to maintain the transport of water, minerals, sugars, and nutrients in the whole plant. 

References

  1. Purcell, Adam. “Xylem and phloem”. Basic Biology. Archivedfrom the original on 2016-05-04.
  2. Keith Roberts, ed. (2007). Handbook of Plant Science. 1(illustrated ed.). John Wiley & Sons. p. 185. ISBN 9780470057230.

Campbell, Neil (2002). Biology. San Francisco, CA: Pearson Education, Inc. pp. 759ISBN978-0-8053-6624-2.